Secondary cell, method for manufacturing secondary cell, positive electrode for secondary cells, method for manufacturing positive electrode for secondary cells, battery pack, electronic device, and electric vehicle

ABSTRACT

A secondary cell with a positive electrode, a negative electrode, and an electrolyte, the positive electrode containing insoluble sulfur.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/201,342 filed Mar. 7, 2014, the entirety of which is incorporatedherein by reference to the extent permitted by law. This applicationclaims the benefit of Japanese Priority Patent Application JP2013-049208 filed Mar. 12, 2013, the entire contents of which areincorporated herein by reference.

BACKGROUND

The present disclosure relates to a secondary cell, a method formanufacturing a secondary cell, a positive electrode for secondarycells, a method for manufacturing a positive electrode for secondarycells, a battery pack, an electronic device, and an electric vehicle.More specifically, the present disclosure relates to a secondary cellthat has a sulfur-containing positive electrode and such a positiveelectrode, methods for manufacturing such a secondary cell and such apositive electrode, and the application of such a secondary cell.

Lithium-sulfur cells, which are secondary cells in which the activematerial for the positive electrode is sulfur, are of public interestbecause this type of secondary cell can have much better rechargeperformance than lithium-ion cells (e.g., see Japanese Unexamined PatentApplication Publication Nos. 2005-251473, 2009-76260, and 2003-197196).A lithium-sulfur cell usually has a positive electrode made ofcrystalline sulfur (or cyclosulfur) (S₈), a negative electrode made ofmetallic lithium, and a non-aqueous electrolyte that contains lithiumions (Li⁺). Sulfur alone is of very low conductivity and is usuallymixed with a conducting agent so that an effective active material canbe obtained. Sulfur can be in a particulate form when mixed with aconducting agent so that continuity can be more easily provided andbetter characteristics can be obtained.

SUMMARY

As mentioned above, the starting material sulfur for such a typicallithium-sulfur cell is usually crystalline sulfur. Crystalline sulfur isobtained through purification processes including sublimation andrecrystallization from carbon disulfide, and the resulting particleshave a large particle diameter. Commercially available crystallinesulfurs, which are manufactured by pulverizing such coarse particles,are as large as about 40 to 100 μm in particle diameter because of thelarge diameter of the initial particles; such commercial products areadditionally ground before use. Furthermore, the positive electrode thatcontains crystalline sulfur is formed from a slurry that contains carbonas a conducting agent in addition to the crystalline sulfur. Such aslurry is difficult to apply because the dispersion of crystallinesulfur in the slurry is often poor. Coatings formed from such a slurryare uneven, and this affects the yield of the coatings. Such coatings,furthermore, contain many aggregate clusters, which make the density ofthe coatings low (e.g., about 0.3 to 0.4 g/cc) and affect the capacityper unit electrode volume of the cell.

It is therefore desirable to provide a secondary cell and a method formanufacturing a secondary cell that both have the following advantages:a simplified and high-yield process for preparing a sulfur-containingpositive electrode, an improved capacity per unit electrode volume, andextremely stable charge-discharge characteristics.

It is also desirable to provide a positive electrode for secondary cellsand a method for manufacturing a positive electrode for secondary cellsthat both have the following advantages: a simplified and high-yieldprocess for producing the positive electrode that can be used when thepositive electrode contains sulfur, an improved capacity per unitelectrode volume, and extremely stable charge-discharge characteristics.

Furthermore, it is desirable to provide a battery pack, an electronicdevice, and an electric vehicle that all use the aforementionedexcellent secondary cell.

An embodiment of the present disclosure is a secondary electrode. Thesecondary cell has a positive electrode, a negative electrode, and anelectrolyte, and the positive electrode contains insoluble sulfur.

Another embodiment of the present disclosure is a method formanufacturing a secondary cell. The method includes applying a slurrythat contains insoluble sulfur to a conductive substrate to form apositive electrode.

Another embodiment of the present disclosure is a positive electrode forsecondary cells. The positive electrode has a conductive substrate andinsoluble sulfur on the conductive substrate.

Another embodiment of the present disclosure is a method formanufacturing a positive electrode for secondary cells. The methodincludes applying a slurry that contains insoluble sulfur to aconductive substrate to form the positive electrode.

Another embodiment of the present disclosure is a battery pack. Thebattery pack has a secondary cell, a control unit for the secondarycell, and a package that contains the secondary cell. The secondary cellhas a positive electrode, a negative electrode, and an electrolyte, andthe positive electrode contains insoluble sulfur.

The control unit of this battery pack controls, for example, the charge,discharge, overcharge, and overdischarge of the secondary cell.

Another embodiment of the present disclosure is an electronic device.The electronic device has a secondary cell and is powered by thesecondary cell. The secondary cell has a positive electrode, a negativeelectrode, and an electrolyte, and the positive electrode containsinsoluble sulfur.

Another embodiment of the present disclosure is an electric vehicle. Theelectric vehicle has a secondary cell and a transducer. The secondarycell has a positive electrode, a negative electrode, and an electrolyte,and the positive electrode contains insoluble sulfur. The transducerreceives electric power from the secondary cell and converts thereceived electrical power into a force that drives the vehicle.

Typically, the transducer for this electric vehicle produces the drivingforce by rotating a motor with the electric power received from thesecondary cell. Regenerated energy can also be used to drive the motor.The control unit carries out tasks such as processing informationconcerning the control of the vehicle depending on the remaining life ofthe secondary cell. Examples of suitable electric vehicles includeelectric automobiles, electric bikes, electric bicycles, and railwaycars and also include hybrid automobiles.

Another embodiment of the present disclosure is an electric powersystem. The electric power system has a secondary cell. The secondarycell has a positive electrode, a negative electrode, and an electrolyte,and the positive electrode contains insoluble sulfur. The electric powersystem is powered by the secondary cell and/or supplies electric powerfrom a power source to the secondary cell.

This electric power system can be any system that handles electric powerand can even be a simple electric power unit. Examples of suitableelectric power systems include smart grids, home energy managementsystems (HEMS), and vehicles. The electric power system can also storeelectrical energy.

Another embodiment of the present disclosure is a power supply for powerstorage. The power supply has a secondary cell and can be connected withthe electronic device to be powered. The secondary cell has a positiveelectrode, a negative electrode, and an electrolyte, and the positiveelectrode contains insoluble sulfur.

Basically, this power supply for power storage can be used with anyelectric power system or electric power unit. For example, the powersupply can be used with a smart grid.

The following describes insoluble sulfur (also referred to as polymericsulfur). Sulfur undergoes gradual structural changes with increasingtemperature. Not only the simple transitions between the three phases,i.e., solid, liquid, and gas, but also changes in crystallographicstructure occur, forming a long-chain structure. This long-chain form ofsulfur is referred to as insoluble sulfur. The term insoluble in“insoluble sulfur” means that the sulfur is insoluble in carbondisulfide (CS₂). Insoluble sulfur can be used as a vulcanizing agent,i.e., an agent used to add sulfur to rubber in a vulcanization process.Crystalline sulfur (S₈) has a ring structure and is soluble in carbondisulfide. In the present disclosure, the positive electrode generallycontains insoluble sulfur and a conducting agent, and typically furthercontains a binder. More specifically, the positive electrode has, forexample, a conductive substrate and a mixture of insoluble sulfur and aconducting agent or a mixture of insoluble sulfur, a conducting agent,and a binder. The conducting agent contains, for example, at least onecarbon material. The at least one carbon material includes, for example,at least one selected from carbon black, activated carbon, carbon fiber,carbon nanotubes, and graphene. Examples of carbon blacks that can beused include Carbon Black #3030B, #3040B, #3050B, #3230B, and #3350B(Mitsubishi Chemical), TOKABLACK #5500, TOKABLACK #4500, TOKABLACK#4400, and TOKABLACK #4300 (Tokai Carbon), Printex L6 and Printex L(Degussa), Conductex 975 and Conductex SC (Columbian Chemicals), VulcanXC 72, Vulcan 9A 32, Black Pearls 2000, and Black Pearls 3700 (Cabot),DENKA BLACK Powder, DENKA BLACK FX-35, and DENKA BLACK HS-100 (DenkiKagaku Kogyo), Ensaco 250G, Ensaco 260G, Ensaco 350G, and Super P-Li(TIMCAL), and KETJENBLACK EC-300J, EC-600JD, ECP, and ECP-600JD (LionCorporation), and KETJENBLACK carbon blacks are preferred. Examples ofactivated carbons that can be used include those made from coal-basedmaterials (e.g., peat, lignite, brown coal, and bituminous coal), plantbiomass materials (e.g., coconut shells, sawdust, rice husks, andlumber), and other materials (e.g., petroleum pitch, plastics(polymers), and organic ashes). Activated carbon is generally producedby carbonizing a starting material and then activating the carbonizedmaterial. Carbonization usually includes heating the carbon, hydrogen,and oxygen in the starting material in an inert gas atmosphere at 400°C. to 700° C. to remove some volatile compounds and subsequentlypreparing a suitable carbide from the heated material for activation.Activation is a process in which the prepared carbide is brought intoreaction at high temperatures of 600° C. to 1000° C. by using steam,carbon dioxide gas, and air so that the remaining volatile compounds andthe carbon atoms in the carbide can gasify and that a porous structuremainly with 10- to 100-Å pores can grow until the internal surface areareaches at least 1000 m²/g. This process provides a porous activatedcarbon. Vapor-grown carbon fibers, a category of carbon fibers, areproduced by carbonizing the starting material acrylic fiber or pitch (aby-product of the production of petroleum, coal, coal tar, etc.) at ahigh temperature, and an example is VGCF (a registered trademark ofShowa Denko K.K.). Examples of binders that can be used includepolyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), styrenebutadiene rubber (SBR), carboxymethylcellulose (CMC), polyamides (PAs),polyamide-imides (PAIS), sodium polyacrylate (PANa),polyvinylpyrrolidone (PVP), polyethylene oxide (PEO),polytetrafluoroethylene (PTFE), polyethylene (PE), polypropylene (PP),and agar. The negative electrode contains, for example, a material thatoccludes and releases lithium ions. More generally, the negativeelectrode contains, for example, at least one selected from lithium,sodium, magnesium, a magnesium salt, aluminum, a lithium-containingalloy, a carbon material capable of occluding and releasing lithiumions, tin oxide, silicon, and titanium oxide. The electrolyte contains,for example, at least one cation selected from lithium, sodium,magnesium, aluminum, and tetraalkylammonium ions.

In some of the foregoing embodiments of the present disclosure, theslurry allows the insoluble sulfur contained therein to be sufficientlymixed at ease because the insoluble sulfur is highly dispersible in theslurry. Coatings formed from the slurry are therefore highly uniform andvery smooth. In the above embodiments, furthermore, industriallyproduced and commercially available fine powders of insoluble sulfur canbe used without additional grinding.

The present disclosure provides a secondary cell that has the followingadvantages: a simplified and high-yield process for preparing asulfur-containing positive electrode, an improved capacity per unitelectrode volume, and extremely stable charge-discharge characteristics.The present disclosure also provides, for example, an electronic device,a battery pack, an electric vehicle, an electric power system, and apower source for power storage that all are of high performance thanksto the use of this excellent secondary cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a cross-sectional view and a plan view,respectively, of a positive electrode for lithium-sulfur cells accordingto Embodiment 1.

FIGS. 2A to 2C are cross-sectional diagrams that illustrate a method formanufacturing a positive electrode for lithium-sulfur cells according toEmbodiment 1.

FIG. 3 is a schematic diagram that illustrates a lithium-sulfur cellaccording to Embodiment 2.

FIG. 4 is a diagram that shows the charge-discharge characteristics ofsome lithium-sulfur cells in which the crystalline-sulfur-containingpositive electrode of the Comparative Example was used.

FIG. 5 is a diagram that shows the charge-discharge characteristics ofsome lithium-sulfur cells in which the insoluble-sulfur-containingpositive electrode of the Example was used.

FIG. 6 is a diagram that shows the charge-discharge characteristics ofsome other lithium-sulfur cells in which the insoluble-sulfur-containingpositive electrode of the Example was used.

FIGS. 7A and 7B are images provided as substitutes for drawings and showthe surface of the electrode of the Comparative Example, which had acoating of a slurry that contained crystalline sulfur, imaged byscanning electron microscopy at the start and end of coating,respectively.

FIGS. 8A and 8B are images provided as substitutes for drawings and showthe surface of the electrode of the Example, which had a coating of aslurry that contained insoluble sulfur, imaged by scanning electronmicroscopy at the start and end of coating, respectively.

FIG. 9 is an image provided as a substitute for a drawing and shows anaggregate cluster and the surrounding area on the surface of theelectrode of the Comparative Example, which had a coating of a slurrythat contained crystalline sulfur, imaged by scanning electronmicroscopy at the end of coating.

FIG. 10 is an image provided as a substitute for a drawing and is amagnified view of the aggregate cluster in FIG. 9 in the area enclosedby the square.

FIG. 11 is an exploded perspective view of a lithium-sulfur cellaccording to Embodiment 3.

FIG. 12 is a cross-sectional view of the wound electrode unit of thelithium-sulfur cell in FIG. 11, taken along line XII-XII.

DETAILED DESCRIPTION OF EMBODIMENTS

The following describes some embodiments of the present disclosure(hereinafter simply referred to as embodiments). The descriptions areprovided in the following order:

1. Embodiment 1

(a positive electrode for lithium-sulfur cells and a method formanufacturing the positive electrode);

2. Embodiment 2

(a lithium-sulfur cell);

3. Embodiment 3

(a lithium-sulfur cell and a method for manufacturing the lithium-sulfurcell).

1. Embodiment 1 Positive Electrode for Lithium-Sulfur Cells

FIG. 1A is a cross-sectional view of a positive electrode forlithium-sulfur cells according to Embodiment 1. FIG. 1B is a plan viewof the same positive electrode for lithium-sulfur cells.

As illustrated in FIGS. 1A and 1B, this positive electrode forlithium-sulfur cells has a conductive substrate 11 and a layer thatcontains insoluble sulfur (an insoluble sulfur layer 12). The insolublesulfur layer 12 typically contains either of the following: insolublesulfur and a conducting agent; or insoluble sulfur, a conducting agent,and a binder. The insoluble sulfur content is preferably 50% by weightor more, e.g., 60% by weight or more; however, these compositions andinsoluble sulfur content parameters are not the only possible options.The conducting agent is an appropriate substance selected from, forexample, the aforementioned materials and is preferably KETJENBLACK. Theinsoluble sulfur layer 12 may contain other appropriate components inaddition to the insoluble sulfur, the conducting agent, and the binder.

The conductive substrate 11 is a substrate made of a conductivematerial. Examples of materials that can be used include, but are notlimited to, metals (pure metals and alloys), conductive oxides, andconductive plastics. Specific examples of metals that can be usedinclude aluminum (Al), platinum (Pt), silver (Ag), gold (Au), ruthenium(Ru), rhodium (Rh), osmium (Os), niobium (Nb), molybdenum (Mo), indium(In), iridium (Ir), zinc (Zn), manganese (Mn), iron (Fe), nickel (Ni),cobalt (Co), titanium (Ti), vanadium (V), chromium (Cr), palladium (Pd),rhenium (Re), tantalum (Ta), tungsten (W), zirconium (Zr), germanium(Ge), and hafnium (Hf) used in the form of a pure metal, a combinationof metals, and an alloy (e.g., stainless steel). The conductivesubstrate 11 may be composed of a non-conductive substrate and aconductive layer. The thickness of the conductive substrate 11 isselected as appropriate and can be in the range of 20 μm to 50 μm, bothinclusive, for example.

Method for Manufacturing the Positive Electrode for Lithium-Sulfur Cells

The following describes an illustrative method for manufacturing thispositive electrode for lithium-sulfur cells.

First, a conductive substrate 11 is provided as illustrated in FIG. 2A.

Then a separately prepared slurry 13 that contains insoluble sulfur isapplied to the conductive substrate 11 as illustrated in FIG. 2B. Atypical composition of the slurry 13 is, for example, insoluble sulfur,a conducting agent (e.g., a carbon powder), a binder (e.g., PVA), and asolvent. Various coating processes can be used to apply the slurry 13;specific examples include knife-over-roll coating, dip coating, spraycoating, wire bar coating, spin coating, roller coating, blade coating,and gravure coating.

Then the conductive substrate 11 is heated with the slurry 13 thereonuntil the slurry 13 dries up and leaves an insoluble sulfur layer 12 asillustrated in FIG. 2C. This heat treatment is carried out using, forexample, a heating furnace. The heating temperature is selected asappropriate and can be in the range of 50° C. to 100° C., bothinclusive, for example. Preferably, an argon (Ar), nitrogen (N₂), orother inert gas atmosphere is used so that the insoluble sulfur and theconducting agent can be prevented from being thermally oxidized.

In such a way, the intended positive electrode for lithium-sulfur cellsis manufactured.

Embodiment 1 therefore provides a novel positive electrode forlithium-sulfur cells that has a conductive substrate 11 and a layer thatcontains insoluble sulfur (an insoluble sulfur layer 12). This positiveelectrode for lithium-sulfur cells can be manufactured by applying aslurry 13 that contains insoluble sulfur to the conductive substrate 11.Compared to coatings formed from a slurry that contains crystallinesulfur, the resulting coating is highly uniform and very smooth and isalso dense; the density of the coating is 0.5 g/cc or more, for example.As a result, the energy density of lithium-sulfur cells isadvantageously improved.

Insoluble sulfur is produced by adding gaseous sulfur to carbondisulfide and allowing sulfur to precipitate. The obtained particles ofinsoluble sulfur have small diameters and are easy to pulverize. Thus,industrially produced fine powders of insoluble sulfur that haveparticle diameters on the order of 4 to 10 μm are commerciallyavailable, and such commercial products can be used without additionalgrinding, unlike crystalline sulfur; the use of insoluble sulfursimplifies processes for manufacturing positive electrodes forlithium-sulfur cells, improving the yield and reducing the manufacturingcost. The use of this positive electrode for lithium-sulfur cells as acomponent of lithium-sulfur cells therefore reduces the cost ofproducing the lithium-sulfur cells.

Another advantage of this positive electrode for lithium-sulfur cells isthat the slurry 13 that contains insoluble sulfur forms very littleaggregate while being applied. The resulting coating is therefore highlyuniform and very smooth. As a result, lithium-sulfur cells that containthis positive electrode for lithium-sulfur cells have extremely stablecharge-discharge characteristics.

2. Embodiment 2 Lithium-Sulfur Cell

The following describes Embodiment 2. In Embodiment 2, a positiveelectrode for lithium-sulfur cells according to Embodiment 1 is used asthe positive electrode of a lithium-sulfur cell, a form of a secondarycell.

FIG. 3 schematically illustrates the basic structure of thislithium-sulfur cell.

As illustrated in FIG. 3, this lithium-sulfur cell has a positiveelectrode 21 and a negative electrode 22 that face each other with anelectrolyte 23 therebetween. A separator is also disposed between thepositive electrode 21 and the negative electrode 22 (not illustrated inFIG. 3). The positive electrode 21 is a positive electrode forlithium-sulfur cells according to Embodiment 1, whereas the negativeelectrode 22 is made of metallic lithium. Materials other than metalliclithium can also be used to make the negative electrode 22, includingcarbon materials capable of occlude and release lithium ions as well astin oxide, silicon, and titanium oxide.

The electrolyte 23 can be a liquid, a gel, or a solid. When theelectrolyte 23 is a gel or a solid, examples of suitable materialsinclude polymers such as polyvinylidene fluoride (PVDF),hexafluoropropylene (HFP), polyvinylidene fluoride-hexafluoropropylene(PVDF-HFP), polyaniline (PAN), and polyethylene oxide (PEO). Copolymerscomposed of such polymers can also be used.

When the electrolyte 23 is an electrolytic solution, the electrolyticsolution can be, for example, a solution of a lithium salt in one or amixture of two or more organic solvents for lithium-ion cells,capacitors, or similar devices. Examples of organic solvents that can beused include the following: carbonates such as ethylene carbonate (EC),propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate(DMC), methyl ethyl carbonate (MEC), and vinylene carbonate (VC); cyclicesters such as γ-butyrolactone (GBL), γ-valerolactone,3-methyl-γ-butyrolactone, and 2-methyl-y-butyrolactone; cyclic etherssuch as 1,4-dioxane, 1,3-dioxolane, tetrahydrofuran,2-methyltetrahydrofuran (MTHF), 3-methyl-1,3-dioxolane, and2-methyl-1,3-dioxolane; and open-chain ethers such as1,2-dimethoxyethane (DME), 1,2-diethoxyethane (DEE), diethyl ether,dimethyl ether, methyl ethyl ether, and dipropyl ether. Other organicsolvents can also be used, including methyl propionate (MPR), ethylpropionate (EPR), ethylene sulfite (ES), cyclohexyl benzene (CHB),tetraphenyl benzene (tPB), ethyl acetate (EA), and acetonitrile (AN).

Examples of lithium salts that can be used in the electrolytic solutioninclude LiSCN, LiBr, LiI, LiClO₄, LiASF₆, LiSO₃CF₃, LiSO₃CH₃, LiBF₄, LiB(Ph)₄, LiPF₆, LiC (SO₂CF₃)₃, and LiN (SO₂CF₃)₂. It is also possible touse a mixture of two or more such lithium salts.

The electrolyte 23 may contain other materials as appropriate forimproved characteristics of the lithium-sulfur cell. Examples ofmaterials that can be used for such purposes include imide salts,sulfonated compounds, and aromatic compounds substituted orunsubstituted with a halogen. Operation of the lithium-sulfur cell

While the lithium-sulfur cell is charging, lithium ions (Li⁺) move fromthe positive electrode 21 to the negative electrode 22 through theelectrolyte 23 and the cell stores electricity by converting electricalenergy into chemical energy. While the lithium-sulfur cell isdischarging, lithium ions return from the negative electrode 22 to thepositive electrode 21 through the electrolyte 23 and the cell produceselectrical energy.

Embodiment 2 provides a novel lithium-sulfur cell that has extremelystable charge-discharge characteristics and can be manufactured at lowcost thanks to the use of a positive electrode for lithium-sulfur cellsaccording to Embodiment 1 as the positive electrode 21.

This lithium-sulfur cell can be installed in driving or auxiliary powersupplies for various equipment and vehicles including the following andcan also be used to power such equipment and vehicles: notebook PCs,PDAs (personal digital assistants), cellular phones, cordless handsets,camcorder/players, digital still cameras, electronic books, electronicdictionaries, mobile music players, radios, headphones, gaming consoles,navigation systems, memory cards, pacemakers, hearing aids, machinetools, electric shavers, refrigerators, air conditioners, TV sets,stereo systems, water heaters, microwave ovens, dishwashers, washingmachines, drying machines, lighting, toys, medical devices, robots, loadconditioners, traffic lights, railway cars, golf carts, mobilityscooters, and electric automobiles (including hybrid automobiles).Likewise, this lithium-sulfur can be installed in power supplies forpower storage for housing and other buildings and for power plants.Electric vehicles have a transducer that receives electric power andconverts the electric power into driving force, and this transducer isgenerally a motor. Electric vehicles also have control units thatprocess information concerning the control of the vehicle, including onethat displays the remaining battery life based on the information aboutthe remaining life of the cell. This lithium-sulfur cell can also beused as an accumulator in electrical grids that are called smart grids.Such an accumulator not only supplies electric power but also storeselectric power when powered by another power source. Examples of powersources that can be used for such purposes include thermal, nuclear, andhydraulic power generation systems, solar cells, wind and geothermalpower generation systems, and fuel cells (including biofuel cells).

EXAMPLE

A positive electrode for lithium-sulfur cells was produced by thefollowing method.

A 20-μm-thick aluminum substrate was used as the conductive substrate11.

A slurry that contained insoluble sulfur was prepared by the followingprocess.

(1) Insoluble sulfur (S) and KETJENBLACK (KB ECP) were mixed in a mortarfor about 30 minutes.

(2) Polyvinyl alcohol (PVA) was dissolved in N-methylpyrrolidone (NMP)on a hot-plate stirrer.

(3) The products of (1) and (2) were weighed in a cup and mixed using acommercially available planetary mixer. The mixing ratios (weightratios) S:KB:PVA were 60:30:10.

(4) NMP was added to make the solid content 20% by weight, and theobtained mixture was mixed using a commercially available planetarymixer.

(5) The mixture was put into a ball-mill pot and blended with beads in aball mill.

(6) The beads were removed to complete the slurry.

The obtained insoluble-sulfur-containing slurry was applied to thesurface of the aluminum substrate by knife-over-roll coating.

Then the coating of the slurry on the surface of the aluminum substratewas dried to form an insoluble sulfur layer that contained insolublesulfur, KETJENBLACK, and PVA. In this way, a positive electrode forlithium-sulfur cells was produced.

Comparative Example

A positive electrode for lithium-sulfur cells was produced by thefollowing method.

A 20-μm-thick aluminum substrate was used as the conductive substrate11.

A slurry that contained crystalline sulfur (S₈) was prepared by aprocess similar to that in the Example.

The obtained crystalline-sulfur-containing slurry was applied to thesurface of the aluminum substrate by knife-over-roll coating.

Then the coating of the slurry on the surface of the aluminum substratewas dried to form a crystalline sulfur layer that contained crystallinesulfur, KETJENBLACK, and PVA. In this way, a positive electrode forlithium-sulfur cells was produced.

Table 1 summarizes the weight, thickness, density, and othermeasurements of the electrode samples of the Example and the ComparativeExample. Samples 1 to 6 were obtained by the method of the ComparativeExample: samples 1 to 3 were produced in a single batch (Lot 1), andsamples 4 to 6 were also produced in a single batch (Lot 2). Samples 11to 16 were obtained by the method of the Example: samples 11 to 13 wereproduced in a single batch (Lot 3), and samples 14 to 16 were alsoproduced in a single batch (Lot 4).

TABLE 1 Active Mixture Average Area Volume material Active ConductingSampling weight thickness density density content material agent Binderpoint [mg] [μm] [mg/cm²] [g/cc] [mg] Sample 1 Crystalli KB ECP PVA Startof 1.96 30.4 1.11 0.365 1.18 Sample 2 ne sulfur coating 1.89 1.07 0.3521.13 Sample 3 (S₈) 1 .96 1.11 0.365 1.18 Sample 4 End of 2.94 40.8 1.660.408 1.76 Sample 5 coating 3.26 1.84 0.451 1.96 Sample 6 3.02 1.710.419 1.81 Sample 11 Insoluble KB ECP PVA Start of 2.34 22.6 1.32 0.5861.40 Sample 12 sulfur (S) coating 2.52 1.42 0.631 1.51 Sample 13 2.41.36 0.601 1.4 Sample 14 End of 2.37 26 1.34 0.516 1.42 Sample 15coating 2.63 1.49 0.573 1.58 Sample 16 2.59 1.47 0.564 1.55

In general, knife-over-roll coating often provides uneven coatingsbecause the slurry forms a settling aggregate while waiting in a dam(i.e., a reservoir) for application. As can be seen from Table 1, theelectrode produced using crystalline sulfur (S₈) was thicker at the endof coating than at the start of coating with the difference in averagethickness as large as about 10 μm, presumably because of the greateramount of aggregate in the slurry at the end of coating. With insolublesulfate (S), the slurry formed less aggregate than that containedcrystalline sulfate (S₈); the increase in average thickness of theelectrode at the end of coating from the start of coating was only about3 μm, demonstrating that the thickness of the electrode was almostconstant. Furthermore, the volume density was higher with insolublesulfur (S) than with crystalline sulfur (S₈).

Then lithium-sulfur cells were produced. Each lithium-sulfur cell wascomposed of the positive electrode for lithium-sulfur cells of theExample or the Comparative Example, a negative electrode made ofmetallic lithium, and an electrolytic solution of 0.5 M LiTFSI and 0.4 MLiNO₃ in DOL/DME. For the positive electrode for lithium-sulfur cells ofthe Comparative Example, two of the samples taken at the start ofcoating (n=2, samples 1 and 2) and two of the samples taken at the endof coating (n=2, samples 4 and 5) were individually used inlithium-sulfur cells of samples 21 to 24. For the positive electrode forlithium-sulfur cells of the Example, the three samples taken at thestart of coating (n=3, samples 11 to 13) and the three taken at the endof coating (n=3, samples 14 to 16) were individually used inlithium-sulfur cells of samples 25 to 30.

The charge-discharge characteristics of the lithium-sulfur cells ofsamples 21 to 30 were then measured. The results are shown in FIGS. 4 to6. FIGS. 4 to 6 present the measurements obtained during the secondcharge-discharge cycle. The discharge current and the charging currentwere 0.1 mA/cm² and 0.3 mA/cm², respectively, for all samples.

When crystalline sulfur (S₈) was used, the charge-discharge capacity wasgreatly different depending on which type of positive electrode wasused, start-of-coating or end-of-coating, as shown in FIG. 4, presumablybecause of the greater amount of aggregate in the end-of-coatingpositive electrode.

When insoluble sulfur (S) was used, however, the charge-dischargecapacity was similar regardless of whether the type of positiveelectrode was start-of-coating or end-of-coating, as shown in FIGS. 5and 6, because insoluble sulfur (S) is highly dispersible in slurry.

FIGS. 7A and 7B are scanning electron microscopic images that show thesurface of the electrode with a coating of the crystalline sulfur(S₈)-containing slurry at the start and end of coating, respectively. Asshown in FIG. 7B, many aggregate clusters that had diameters of 20 to100 μm were observed at the end of coating.

FIGS. 8A and 8B are scanning electron microscopic images that show thesurface of the electrode with a coating of the insoluble sulfur(S)-containing slurry at the start and end of coating, respectively. Ascan be seen from FIGS. 8A and 8B, aggregate clusters were few in numberboth at the start and end of coating.

FIG. 9 is a scanning electron microscopic image that shows the surfaceof the electrode that had a coating of the crystalline sulfur(S₈)-containing slurry at the end of coating, close up to one aggregatecluster. FIG. 10 is a scanning electron microscopic image that shows amagnified view of the aggregate cluster in FIG. 9 in the area enclosedby the square (a pore). Irradiating the surface of the electrode in FIG.10 with an electron beam from the electron microscope easily evaporatedthe aggregate, suggesting that the main component of the aggregate wassulfur.

The arithmetic mean surface roughness (R_(a)) of these electrodes wasevaluated using a laser scanning microscope (Keyence). The results aresummarized in Table 2. As can be seen from Table 2, the R_(a) of thecrystalline sulfur (S₈)-containing electrode measured at the end ofcoating was greatly different from that at the start of coating; thesurface of the electrode was much rougher at the end of coating,presumably because of the greater amount of aggregate at the end ofcoating. The insoluble-sulfur-containing electrode exhibited similarR_(a) values at the start and end of coating compared with thatcontained crystalline sulfur (S₈); the thickness of this electrode wassubstantially constant.

TABLE 2 Arithmetic mean Test articles surface roughness (R_(a)) S₈,start of coating  4.25 μm S₈, end of coating 12.04 μm Insoluble S,  5.46μm start of coating Insoluble S,  6.56 μm end of coating

3. Embodiment 3 Lithium-Sulfur Cell

In Embodiment 3, a specific example of the structure of a lithium-sulfurcell according to Embodiment 2 is described.

FIG. 11 is an exploded perspective view of this lithium-sulfur cell.

As illustrated in FIG. 11, this lithium-sulfur cell has a woundelectrode unit 33 and film package components 34 a and 34 b, and thewound electrode unit 33 has lead wires on the positive side and thenegative side (a positive electrode lead 31 and a negative electrodelead 32, respectively).

The positive electrode lead 31 and the negative electrode lead 32 extendout of the package components 34 a and 34 b and are headed in, forexample, the same direction. Examples of the materials of which thepositive electrode lead 31 and the negative electrode lead 32 areindividually made include metals such as aluminum (Al), copper (Cu),nickel (Ni), and stainless steel. The positive electrode lead 31 and thenegative electrode lead 32 can be, for example, thin plates or pieces ofwire mesh.

The package components 34 a and 34 b can be, for example, rectangularpieces of a laminated film composed of nylon film, aluminum foil, andpolyethylene film stacked in this order. In an illustrative structure,the package components 34 a and 34 b face the wound electrode unit 33 onthe polyethylene film side and are in contact with each other at theedge by fusion bonding or with an adhesive agent. An adhesive film 35 isinterposed between each of the package components 34 a and 34 b and eachof the positive electrode lead 31 and the negative electrode lead 32 tokeep out the external air. The adhesive film 35 is made of a materialthat adheres to the positive electrode lead 31 and the negativeelectrode lead 32. For example, when the positive electrode lead 31 andthe negative electrode lead 32 are made of any of the metals listedabove, the adhesive film 35 is preferably made of polyethylene,polypropylene, modified polyethylene, modified polypropylene, or anyother polyolefin.

The package components 34 a and 34 b can also be made from materialsother than the laminated film described above, e.g., a laminated film ina different structure, a film of polypropylene or other polymers, and ametal film.

FIG. 12 is a cross-sectional view of the wound electrode unit 33 in FIG.11, taken along line XII-XII.

As illustrated in FIG. 12, the wound electrode unit 33 is a stack of apositive electrode 21 and a negative electrode 22 wound with a separator36 and an electrolyte 23 therebetween and is wrapped in a protectivetape 37.

In an illustrative structure, the positive electrode 21 has a collector(a positive electrode collector 21 a) that has a pair of opposite sidesand a compound layer (a positive electrode compound layer 21 b) on bothsides or either side of the positive electrode collector 21 a. Thepositive electrode collector 21 a is exposed, i.e., not covered with thepositive electrode compound layer 21 b, in a portion at eitherlongitudinal end, and the positive electrode lead 31 extends from thisexposed portion. The positive electrode collector 21 a corresponds tothe conductive substrate 11 of the positive electrode for lithium-sulfurcells illustrated in FIG. 1, and examples of the materials from whichthis collector can be made include metal foils such as aluminum foil,nickel foil, and stainless steel foil. The positive electrode compoundlayer 21 b corresponds to the carbon nanotubes 12 and the sulfur 13 onthe conductive substrate 11 of the positive electrode for lithium-sulfurcells illustrated in FIG. 1.

The negative electrode 22, in an illustrative structure, has a collector(a negative electrode collector 22 a) that has a pair of opposite sidesand a compound layer (a negative electrode compound layer 22 b) on bothsides or either side of the negative electrode collector 22 a. Thenegative electrode collector 22 a is preferably made from copper (Cu)foil, nickel foil, stainless steel foil, or any other metal foil thathas good electrochemical stability, conductivity, and mechanicalstrength. In particular, copper foil is highly preferred because of highconductivity. The negative electrode compound layer 22 b is made of, forexample, metallic lithium.

The separator 36 is, for example, a plastic or ceramic porous film, andexamples of plastic materials that can be used includepolytetrafluoroethylene, polypropylene, and polyethylene. A multilayerporous film that has two or more porous layers made of different plasticor ceramic materials can also be used. Polyolefin porous films arepreferred because such a film effectively prevents short-circuiting andimproves the safety of the cell by the “shut-down” effect. Inparticular, polyethylene is a highly preferred material for theseparator 36 because of the shut-down effect that occurs in thetemperature range of 100° C. to 160° C., both inclusive, and excellentelectrochemical stability. Polypropylene is also preferred. Copolymersor blends of polyethylene or polypropylene with other chemically stableplastics can also be used. Method for manufacturing the lithium-sulfurcell

The following describes an illustrative method for manufacturing thislithium-sulfur cell.

First, a positive electrode compound layer 21 b is formed on a positiveelectrode collector 21 a to form a positive electrode 21, and a negativeelectrode compound layer 22 b is formed on a negative electrodecollector 22 a to form a negative electrode 22.

Then, in an illustrative procedure, a positive electrode lead 31 isattached to the positive electrode collector 21 a, and an electrolyte 23is formed on the positive electrode compound layer 21 b, i.e., on bothsides or either side of the positive electrode 21. Likewise, a negativeelectrode lead 32 is attached to the negative electrode collector 22 a,and the electrolyte 23 is formed on the negative electrode compoundlayer 22 b, i.e., on both sides or either side of the negative electrode22.

After the electrolyte 23 is formed, the positive electrode 21 and thenegative electrode 22 are stacked. The obtained stack is wound and thenwrapped in a protective tape 37 to form a wound electrode unit 33.

After the wound electrode unit 33 is formed in such a way, in anillustrative procedure the wound electrode unit 33 is sandwiched betweenpackage components 34 a and 34 b and sealed by joining the packagecomponents 34 a and 34 b at the edge by fusion bonding or other suitabletechniques, with an adhesive film 35 placed between each of the positiveelectrode lead 31 and the negative electrode lead 32 and each of thepackage components 34 a and 34 b.

By such a method, the lithium-sulfur cell illustrated in FIGS. 11 and 12is manufactured.

The advantages of Embodiment 3 are similar to those of Embodiment 2.

The foregoing is a detailed description of some embodiments and anexample of the present disclosure. It should be understood that thepresent disclosure is not limited to the foregoing embodiments andexample and various modifications may occur.

For instance, the values, structures and configurations, shapes, andmaterials mentioned in the foregoing embodiments and example are forillustration purposes only. Different values, structures andconfigurations, shapes, and materials may be used as appropriate.

To take an example, the cell can be in forms other than a wound cell,such as a multilayer cell and a bi-cell. The bi-cell can be, forexample, a form of a cell that has unit A (a stack of a positiveelectrode/an electrolyte/a separator/an electrolyte/a negativeelectrode/an electrolyte/a separator/an electrolyte/a positiveelectrode), unit B (a structure that has a positive electrode in placeof a negative electrode and vice versa, i.e., a stack of a negativeelectrode/an electrolyte/a separator/an electrolyte/a positiveelectrode/an electrolyte/a separator/an electrolyte/a negativeelectrode), and a strip separator folded over in several layers withunits A and B alternately interposed.

Furthermore, the negative electrode may contain a material that iscapable of occluding and releasing ions other than lithium ions, unlikethat in the lithium-sulfur secondary cells described as some embodimentsof the present disclosure. For example, materials capable of occludingand releasing ions such as sodium, magnesium, magnesium salt, andaluminum ions can be used to make the negative electrode. When such anegative electrode is used, the electrolyte can be an electrolyte thatcontains one or a combination of cations selected from sodium,magnesium, aluminum, and tetraalkylammonium ions.

The present technology can take other forms including the following:

(1) a secondary cell that has a positive electrode, a negativeelectrode, and an electrolyte, the positive electrode containinginsoluble sulfur;

(2) the secondary cell according to (1), wherein the positive electrodecontains the insoluble sulfur and a conducting agent;

(3) the secondary cell according to (2), wherein the conducting agentcontains at least one carbon material;

(4) the secondary cell according to (3), wherein the at least one carbonblack material includes at least one selected from carbon black,activated carbon, carbon fiber, carbon nanotubes, and graphene;

(5) the secondary cell according to any of (1) to (4), wherein thenegative electrode contains a material that occludes and releaseslithium ions;

(6) the secondary cell according to any of (1) to (4), wherein thenegative electrode contains at least one selected from lithium, sodium,magnesium, a magnesium salt, aluminum, a lithium-containing alloy, acarbon material capable of occluding and releasing lithium ions, tinoxide, silicon, and titanium oxide;

(7) the secondary cell according to any of (1) to (7), wherein theelectrolyte contains at least one cation selected from lithium, sodium,magnesium, aluminum, and tetraalkylammonium ions;

(8) a method for manufacturing a secondary cell, the method includingapplying a slurry that contains insoluble sulfur to a conductivesubstrate to form a positive electrode;

(9) the method for manufacturing a secondary cell according to (8),wherein the slurry contains the insoluble sulfur and a conducting agent;

(10) a positive electrode for secondary cells, the positive electrodehaving a conductive substrate and insoluble sulfur on the conductivesubstrate;

(11) a method for manufacturing a positive electrode for secondarycells, the method including applying a slurry that contains insolublesulfur to a conductive substrate to form the positive electrode;

(12) a battery pack that has a secondary cell, a control unit for thesecondary cell, and a package that contains the secondary cell, thesecondary cell having a positive electrode, a negative electrode, and anelectrolyte, the positive electrode containing insoluble sulfur;

(13) an electronic device that has a secondary cell, the secondary cellhaving a positive electrode, a negative electrode, and an electrolyte,the positive electrode containing insoluble sulfur, the electronicdevice powered by the secondary cell; and

(14) an electric vehicle that has a secondary cell and a transducer, thesecondary cell having a positive electrode, a negative electrode, and anelectrolyte, the positive electrode containing insoluble sulfur, thetransducer configured to receive electrical power from the secondarycell and convert the received electrical power into a force that drivesthe vehicle.

What is claimed is:
 1. A battery comprising: a positive electrodeincluding insoluble sulfur, a conductive agent and a binder; a negativeelectrode; and an electrolyte including at least one ofhexafluoropropylene (HFP), polyvinylidene fluoride-hexafluoropropylene(PVDF-HFP), polyaniline (PAN), polyethylene oxide (PEO), or a copolymerof any of them.
 2. The battery according to claim 1, wherein theconductive agent including carbon material.
 3. The battery according toclaim 2, wherein the carbon material is at least one of carbon black,activated carbon, carbon fiber, a carbon nanotube, or graphite.
 4. Thebattery according to claim 1, wherein the negative electrode includes atleast one of lithium, sodium, silicon, aluminum, magnesium, a magnesiumsalt, a lithium-containing alloy, a carbon material, tin oxide, ortitanium oxide.
 5. The battery according to claim 1, wherein theelectrolyte includes at least one of a lithium ion, a sodium ion, amagnesium ion, an aluminum ion, or a tetraalkylammonium ion.
 6. Abattery pack comprising: a battery according to claim 1; a control unitfor the battery; and a package containing the battery.
 7. An electronicdevice powered by a battery according to claim
 6. 8. A electric vehiclecomprising: a battery according to claim 1, a transducer configured toreceive electrical power from the battery and convert the receivedelectrical power into a force that drives the electric vehicle.